1. Field of the Invention
The present invention relates generally to epoxy-based resins and more specifically to epoxy based resins having little shrinkage or stress upon curing.
2. Description of the Prior Art
The formation of polymers from epoxy resins is accompanied by a reduction in the volume of the reactants. This shrinkage is unavoidably created when atoms which are separated from one another as monomers by the sum of their van der Waals radii come together to assume a covalent bond distance in polymers. This shrinkage creates internal strain in the resin, and may cause it to pull away from reinforcing fibers, fillers, or its substrate, thus creating loss of adhesion and voids or channels for the penetration of foreign and potentially destructive substances. Shrinkage creates internal strain and diminishes the mechanical properties of resins. The resulting loss in performance becomes especially important for resins designed for exposure at high temperature, or for cycling between low and high temperatures.
Polymerization which proceeds via cationic and free-radical mechanisms with some net expansion was discovered by William J. Bailey [See for example W. J. Bailey in Ring-Opening Polymerizations, J. E. McGrath, Ed., ACS Symposium Series 286, pp. 47-65, American Chemical Society, Washington, D.C., 1985.] Anionic polymerizations with expansion were discovered by Brady and Simon [Journal of Polymer Science, Polymer Chemistry Edition, 25, 231-239 (1988)], but the monomers they used have useful lives measured in weeks, and are not practical materials in a manufacturing environment.
The use of acid anhydrides to cure epoxy resins is well known. [See Lee and Neville, Handbook of Epoxy Resins, Chapter 23, McGraw Hill, N.Y., 1967.] Customarily, a mixture of an epoxy resin and an anhydride is cured at 120.degree. to 130.degree. C., with or without a catalyst, for 20 to 24 hours. Sublimation of the anhydride at curing temperatures, and crystallization of the anhydride from the epoxy resin at room temperature are obstacles to the convenient use of anhydrides.